1. Field of the Invention
The present invention relates to a method of generating a correlation function in which side-peaks have been eliminated from an autocorrelation function in order to track binary offset carrier (BOC) signals, and a system for tracking a BOC signal using the method. More particularly, the present invention relates to a method of eliminating side-peaks from every type of BOC signal regardless of the type of phase of the sub-carrier of a BOC signal identified as BOCsin(kn,n) or BOCcos(kn,n) and the value of k, and a system for tracking a BOC signal using the method, where k is the ratio of the chip period Tc of the pseudorandom noise (PRN) code to the period of the sub-carrier and n denotes the ratio of Tc−1 to 1.023 MHz
2. Description of the Related Art
BOC modulation is scheduled to be used for next generation global navigation satellite systems (GNSSs), such as Galileo and the modernized global positioning system (GPS).
In navigation satellite systems, a time synchronization error is directly related to a distance measurement error, and thus the accurate synchronization of a BOC signal is significantly important to the implementation of reliable navigation satellite systems.
BOC modulation has the characteristic of shifting energy from the center of a band to the side thereof, unlike a binary phase shift keying (BPSK) method that is used in a GPS. This characteristic allows the BOC-based global navigation satellite system (GNSS) to share a spectrum with existing signals, thereby enabling the GPS and Galileo to achieve frequency band sharing. In addition, due to the characteristic of the sharpness of the peak of the BOC autocorrelation, more accurate positioning can be performed on the assumption that accurate synchronization has been established. Precise synchronization is required to utilize the above-described advantages of BOC modulation. Signal synchronization is performed based on a peak of the autocorrelation. Although other existing signals do not cause any problem because they produce a single peak, a BOC signal undergoes difficulty performing synchronization because multiple peaks occur in the autocorrelation.
One of the most important problems of Galileo is a BOC signal tracking problem. Since the autocorrelation of a BOC signal has multiple side-peaks, there is a concern about converging on a false lock point or entering an unstable state in a tracking stage. In order to deal with this problem, various researches have been conducted.
In order to overcome the multiple side-peak problem of a BOC signal, Julien proposed a method of eliminating side-peaks by subtracting the cross-correlation between a BOC signal and a PRN signal from BOC autocorrelation. Unfortunately, this technique is applicable only to BOCsin(n,n) (see O. Julien, C. Macabiau, M. E. Cannon, and G. Lachapelle, “ASPeCT: unambiguous sine-BOC(n,n) acquisition/tracking technique for navigation applications,” IEEE Trans. Aer. Electron. Syst., vol. 43, no. 1, pp. 150-162, January 2007).
The scheme proposed by Burian eliminates some of the side-peaks, but does not overcome the ambiguity problem (see A. Burian, E. S. Lohan, and M. K. Renfors, “Efficient delay tracking methods with sidelobes cancellation for BOC-modulated signals,” EURASIP Journ. Wireless Commun. Network., vol. 2007, article ID. 72626, 2007).
The scheme proposed by Kim et al. completely eliminates side-peaks, but is problematic in that it exhibits worse signal tracking performance for k>1 compared to the BOC autocorrelation function (see S. Kim, D. Chong, and S. Yoon, “A new GNSS synchronization scheme,” in Proc. Vehic. Technol. Conf. VTC, CD-ROM, Barcelona, Spain, April 2009).
Recently, a new scheme referred to as a pseudo correlation function based unambiguous delay lock loop (PUDLL) was proposed (see Z. Yao, X. Cui, M. Lu, Z. Feng, and J. Yang, “Pseudo-correlation-function-based unambiguous tracking technique for sine-BOC signals,” IEEE Trans. Aer. Electron. Syst., vol. 46, no. 4, pp. 1782-1796, October 2010). This scheme achieves improved signal tracking performance for a BOC signal in which the value of k is small compared to BOC autocorrelation while completely eliminating side-peaks, using cross-correlation between a received BOC signal and specially designed local signals. However, this scheme is problematic in that as the k value increases, the performance thereof becomes increasingly worse than that of the BOC autocorrelation. Furthermore, this scheme is limited in that it is applicable only to BOCsin (kn,n) in which k is an integer.
As a result, there is a demand for a method of eliminating side-peaks that can be universally applied regardless of the type of phase of a BOC signal and the value of k.